Aligned nanostructured polymers

ABSTRACT

Novel, simple methods are presented directed to the synthesis of aligned nanofibers of polyaniline and substituted derivatives on a substrate. The production of these fibers is achieved via various methods by controlling the concentration of aniline monomer or substituted aniline derivatives or an oxidant in the reaction medium and maintaining said concentration at a level much lower than conventional polyaniline synthesis methods. Methods are disclosed relating to the use of a permeable membrane to control the release of a monomer and/or oxidant as well as a bulk polymerization method.

This application is a divisional of U.S. application Ser. No.11/731,974, filed on Apr. 2, 2007 now U.S. Pat. No. 8,038,907, which isa continuation-in-part of U.S. application Ser. No. 11/168,751, filed onJun. 28, 2005 now U.S. Pat. No. 7,374,703, and claims the benefit ofU.S. Provisional Application No. 60,788,609, filed on Apr. 3, 2006.

BACKGROUND

The present exemplary embodiments relate to the synthesis of polyanilineand its substituted derivatives. It finds particular application inconjunction with the synthesis of aligned electrically conductive andnon-conductive polyaniline nanofibers, and will be described withparticular reference thereto. However, it is to be appreciated that thepresent exemplary embodiment is also amenable to other likeapplications.

Electroactive conductive polymers have been subject to extensiveresearch in recent years. Polymers which show electrical conductivitydue to the structure of the polymeric chain may be used to replace metalconductors and semiconductor materials in many applications. Significantapplications include providing a conductive pathway in circuits anddevices, displays, lighting, chemical, biological, environmental andmedical sensors, anticorrosive coatings, scaffolds for tissue growth,antistatic shielding (ESD) and electromagnetic shielding (EMI).

In the group of intrinsically electrically conductive polymers, onetechnically promising polymer is polyaniline. Polyaniline has emerged asone of the most promising conducting polymers and can be used in avariety of applications, such as paint, antistatic protection,electromagnetic radiation protection, electro-optic devices such asliquid crystal devices (LCDs), light emissive displays, lighting andphotocells, transducers, circuit boards, chemical, biological,environmental and medical sensors, anticorrosive coatings, scaffolds fortissue growth, etc.

Polyaniline is one of a class of conductive polymers, which can besynthesized through either chemical polymerization or electrochemicalpolymerization. Polyaniline is conventionally prepared by polymerizingan aniline monomer. The nitrogen atoms of monomer units are bonded tothe para-carbon in the benzene ring of the next monomer unit. Inchemical preparation, bulk polymerization is the most common method tomake polyaniline. As has been previously reported, conventional bulkchemical synthesis produces granular polyaniline.

Thus, a variety of other chemical methods have been used in order toobtain polyaniline nanofibers. These approaches include use of templatesor surfactants, electrospinning, coagulating media, interfacialpolymerization, seeding, and oligomer-assisted polymerization. Amongthese methods, interfacial polymerization is perhaps the easiest andleast expensive means to obtain nanofibers in one step. However, thismethod requires organic solvents to dissolve the aniline monomer,resulting in a waste stream that must be treated.

It is known that a thin film of polyaniline is deposited on substratesin the conventional polymerization of aniline, called in-situ adsorptionpolymerization. However, the deposited thin film prepared by theconventional chemical polymerization is only composed of irregulargranular particulates. Until now, aligned and oriented nanostructures ofpolyaniline are generally produced in chemical or electrochemicalpolymerization through the assistance by hard templates. In thehard-template polymerization, polyaniline is confined to growth insidethe channels of the membranes. After polymerization, the templates haveto be removed or etched away carefully to obtain an orientednanostructured thin film. The diameters of the aligned nanofibers arelimited by the sizes of the pores of the template membranes used.Recently, a step-wise electrochemical deposition process was introducedto deposit oriented polyaniline nanofibers on the conductive substrates(e.g. Au, Pt etc.) without using a hard template. This method is limitedby the substrates. It is necessary to use electrically conductivematerials for substrates to obtain oriented nanostructures. For templateor step-wise electrochemical deposition methods, a very large uniformarray of aligned and oriented nanostructures is generally not possibleto fabricate (e.g. a letter sized substrate, 8.5 inches×11 inches).Those limitations restrict the applicability of the aligned nanofibers,especially for use in surface response (e.g., superhydrophobic orsuperhydrophilic surfaces), electrodes for organic or polymericlight-emitting diodes, field emission display, DNA stretching, chemicalsensors, biosensors etc.

Therefore, there is an interest in devising an easy, inexpensive,environmentally friendly and scalable one-step method to produce highlypure, uniform nanofibers with controllable average diameters rangingfrom 5 nm to 250 nm in bulk quantities to meet the requirements forpotential use on cell culture, electronic devices, sensors, biosensors,supercapacitors, hydrogen storage, etc.

INCORPORATION BY REFERENCE

The disclosures of U.S. application Ser. No. 11/731,974, filed on Apr.2, 2007, U.S. application Ser. No. 11/168,751, filed on Jun. 28, 2005,and U.S. Provisional Application No. 60/788,609, filed on Apr. 3, 2006,are each incorporated herein by reference in their entirety.

BRIEF DESCRIPTION

In a first aspect, there is provided a process for Application No.forming aligned nanofibers of polyaniline or a substituted polyanilineon a substrate including the steps of: providing a first solutioncontaining aniline monomer or substituted aniline monomer; providing asecond solution containing an oxidant, wherein at least one of the firstand second solutions further contains an acid; providing a permeablemembrane separating the first and second solutions, wherein the membranepermits at least one of the monomer and th

BRIEF DESCRIPTION

In a first aspect, there is provided a process for forming alignednanofibers of polyaniline or a substituted polyaniline on a substrateincluding the steps of: providing a first solution containing anilinemonomer or substituted aniline monomer; providing a second solutioncontaining an oxidant, wherein at least one of the first and secondsolutions further contains an acid; providing a permeable membraneseparating the first and second solutions, wherein the membrane permitsat least one of the monomer and the oxidant to pass therethrough at acontrolled rate; allowing at least one of the monomer and the oxidant topass through the membrane to form a reaction solution of monomer andoxidant; providing a substrate; and polymerizing monomer in the reactionsolution to form aligned polymer nanofibers on a surface of thesubstrate.

In a second embodiment, aligned polyaniline nanofibers can be obtainedfrom conventional bulk chemical polymerization under careful control ofpolymerization conditions. This is accomplished by introducing anilinemonomer solution into an oxidant solution (or vice versa) andpolymerizing at low concentrations and depositing said alignednanofibers on a substrate.

In a third embodiment, there are provided various applications ofaligned polyaniline fibers deposited on a substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the nanostructure of sulfite ion functionalized polyanilinefibers both before and after nucleophilic addition.

FIG. 2 shows SEM images for sulfonated polyaniline synthesized fromdifferent concentrations of the monomers (aniline and3-aminobenzenesulfonic acid).

FIG. 3 is a transmission electron micrograph (TEM) of polyanilinenanofibers obtained in different dopant acids (a) CSA (b) CH₃SO₃H (c)HClO₄, with the inset image showing a different area with nanofibers oflarger diameters.

FIG. 4 is Scanning electron micrograph (SEM) of polyaniline nanofibersobtained from different dopant acids (a) camphor sulfonic acid (CSA) (b)CH₃SO₃H (c) HClO₄.

FIG. 5 shows a scanning electron micrograph (SEM) of polyanilinenanofibers obtained in 1M HCl_((aq)). (a)-(b) conventionalpolymerization: [aniline]=0.4M, (c)-(d) dilute polymerization:[aniline]=0.008M.

FIG. 6 is a UV/vis spectra of polyaniline/HClO₄ nanofibers dispersed indeionized water after purification by rinsing with deionized water(dotted line), after subsequent addition of a drop of diluteHClO_(4(aq)) (dashed line) and after further adding a drop of 30% w/wNH₄OH_((aq)) (solid line).

FIG. 7 is an X-ray diffraction pattern of the film formed by castingdoped polyaniline/HClO₄ nanofibers from deionized water dispersion.

FIG. 8(A)-(C) are scanning electron micrographs (SEMs) of polyanilinenanofibers deposited on a Si-wafer substrate with a thin layer coatingof Au/Pd. (A) Low magnification, ×100; scale bar=200 μm (B) Highmagnification, ×20,000; scale bar=1 μm (C) Isolated single nanofibers;magnification, ×10,000; scale bar=2 μm).

FIGS. 8(D and E) are transmission electron micrograph (TEM) ofpolyaniline nanofibers dispersed in ethanol under ultrasonic bath, andthen deposited on copper grid substrate. (scale bar=(D) 200 nm (E) 50nm).

FIG. 9 are X-ray diffraction (XRD) patterns of polyaniline nanofibers.After purification, as-prepared polyaniline nanofibers are onlypartially doped by HClO_(4(aq)). The sample is equilibrated with 0.1MHClO_(4 (aq)) (labeled as ES-ClO₄) (A). After purification (and withouttreatment by 0.1M HClO_(4(aq))), the as-prepared sample is dedoped by0.1M NH₄OH_((aq)), and then washed by deionized water to form emeraldinebase (EB) (B). After XRD measurement of the sample (B), it is exposed toHCl_((v)) vapor for 30 minutes (labeled as ES-Cl-vapor) (C). A secondportion of sample B (undoped polyaniline nanofibers, EB) is redoped by0.5M HCl_((aq)) and then washed by dialysis against deionized water(labeled as ES-Cl-p) (D). All dispersed samples are deposited on Rigakuglass powder holder for measurement. X-ray patterns were taken with CuKαradiation (λ=1.54059292 Å).

FIG. 10 are UV/vis spectra of polyaniline nanofibers. (A) Emeraldinebase form (EB) of polyaniline nanofibers. Note that the unusually broadnature of the peak at 500-900 nm suggests greater planarity of thepolymer chains and/or the possible presence of residual dopants in thesenanofibers. (B) Undoped polyaniline nanofibers (EB) was redoped by 0.5MHCl_((aq)), and then washed by deionized water to form the dispersion ofthe partially doped emeraldine salt, termed ES-Cl-p. (C) ES-Cl-pprecipitate was redispersed in a large amount of deionized water forUV/vis study, resulting in the partial removal of counter ion within thepolymer backbone to form ES-Cl-w. (D) Addition of a droplet of aqueous37% w/w HCl_((aq)) to the above dispersion, ES-Cl-w results in fullydoped polyaniline nanofibers, ES-Cl.

FIG. 11 are Scanning electron micrographs (SEM) of aligned polyanilinenanofibers grown on the over-head PET transparency with surface-coatingremoved. (a) Top view, low magnification. (b)-(c) Top view, highmagnification. (d)-(e) Tilted view, high magnification. (f) Tilted viewfor the edge of the partial removed thin film. Polymerizationconditions: [Aniline]=0.01M, [Aniline]/[APS]=1.5, Temperature=0˜5° C.(ice bath), stirring reaction, and [HClO₄]=1M.

FIG. 12 is a tilt-viewed scanning electron micrograph (SEM) of alignedpolyaniline nanofibers grown on an over-head PET transparency withsurface coating un-removed. The left-upper notation is the initialconcentration of aniline used for polymerization, where mM=10⁻³ M. Themolar ratio of aniline to APS is fixed at 1.5. All polymerization iscarried out and in the ice bath (0˜5° C.) and [HClO₄]=1M. (The scalebar=500 nm)

FIG. 13 is a UV/vis spectra of aligned and oriented polyanilinenanofibers grown on the over-head PET transparency with surface-coatingremoved. The notations on the spectra are the initial concentration ofaniline used for polymerization, where mM=10⁻³ M. Polymerizationconditions: [Aniline]=shown on the spectra, [Aniline]/[APS]=1.5,Temperature=0˜5° C. (ice bath), stirring reaction, and [HClO₄]=1M.

FIG. 14 is a graph of UV/vis absorption intensities as a function ofconcentration of aniline.

FIG. 15 shows the surface roughness effect of the (a) hydrophobic and(b) hydrophilic surfaces.

FIG. 16 shows aligned polyaniline nanofibers deposited on theline-patterning over-head transparency film. The uncoated area resultsfrom the removal of the printer toner.

FIG. 17 is a graph of the operation voltage of an ITO electrode coatedwith a layer of aligned polyaniline as a function of current in aITO/PAN nanofiber/Alq₃/Ca/Al Organic Light Emitting Device structure.

FIG. 18 is a graph showing the conductance change of nanofibers uponapplying gate voltages in a field effect device based on polyanilinenanofibers.

DETAILED DESCRIPTION

As stated above, the present exemplary embodiments are directed to thesynthesis of aligned polyaniline nanofibers. According to the classicaltheory of nucleation and growth, the mechanism of nucleation followed bygrowth is responsible for polyaniline morphology. An elongated form(e.g., fibers, tubes or rods) is established as the growth rate forpolyaniline is distinctly not identical in all directions (anisotropicgrowth). Moreover, new active nucleation centers can occur on initiallyformed nanofibrils for additional growth of nanofibrils resulting in abranched network or dendrites (secondary nucleation). If reducedconcentrations of monomer and/or oxidant are used or the concentrationsof monomer and/or oxidant are careful controlled in the initial stage ofpolymerization, polyaniline may form in favor of 1-D nanostructuresexclusively. Following this route, two novel, facile methods areintroduced to synthesize polyaniline nanofibers.

In the parent application, two approaches were introduced to synthesizepolyaniline nanofibers in an aqueous solution without aid of specifictemplates, such as surfactants, large organic dopant acids, organicsolvents, nanoscale seeds, oligomers, etc. In the first approach (calleddilute polymerization), polymerization needs to be carried out in diluteaniline in the presence of the protonic acid. This contrasts with therelatively high concentrations of aniline used to synthesize polyanilinepowders in conventional polymerization methods. The term “diluteaniline” is used here to describe the polymerization of aniline beingcarried out at a substantially lower concentration of aniline, e.g. lessthan ˜0.015M, compared to the one used in the conventional synthesis,e.g. ˜0.435M of aniline. In addition, the molar ratio of aniline tooxidant in the dilute polymerization process is in the range of 10:1 to0.1:1. This contrasts to the fixed molar ratio of aniline to oxidant of4.35:1 in the conventional synthesis.

In the second approach (called porous membrane controlledpolymerization), a permeable tubing or membrane is used to steadilycontrol the release of aniline monomer into an oxidant solution (or viceversa) in the protonic acid media to form polyaniline nanostructures.After polymerization, polyaniline nanofibers are collected directlyoutside the tubing or inside the tubing without any further treatment toobtain free-standing nanofibers.

It has now been determined that In addition to the nanofibers formed inthe bulk solution, aligned and molecularly ordered polyanilinenanofibers (or nanowires) also can be formed simultaneously on anysubstrates (e.g. the wall of the beaker, the interface between air andreaction solution, solid materials, stirring magnetic bar, etc.) presentin the reaction mixture using either of the above dilute polymerizationor porous membrane controlled polymerization processes.

The present embodiments show the ability to successfully coat alignedand oriented nanofibers (or nanowires) of polyaniline and itsderivatives on a variety of sized substrates (e.g. submicron PET fibersup to a letter sized over-head transparency) using the novel, twomethods, dilute polymerization and porous membrane controlledpolymerization. There is no apparent difference in the coatingmorphology of the aligned nanofibers on either conductive (e.g. ITO) orinert substrates (PET, PMMA, PS, PDMS, etc.) for these two methods. Inaddition, the geometry (e.g. flat or hierarchical surfaces) of thesubstrates has no effect on the coating. This shows that the two methodswe propose are very robust and this also leads to broad application ofthis aligned and oriented nanostructure.

The production of these aligned fibers is achieved via various methodsby introducing a substrate while controlling the concentration ofaniline monomer or an oxidant in the reaction medium and maintainingsaid concentration at a level much lower than conventional polyanilinesynthesis methods. Although not intended to be limiting, excellentresults are achieved with a concentration of monomer in a reactionsolution of 10 millimoles or less. This control can be accomplished byvarious methods.

In a first embodiment, termed “porous membrane controlledpolymerization” as described above, aniline monomer dissolved in anaqueous acid solution is separated from an aqueous oxidant/acid solutionby a permeable membrane in a reaction chamber containing a substrate.The aniline monomer diffuses through the membrane at a controlled rateand is subsequently polymerized in the oxidant/acid solution accordingto known reactions. Alternately or in addition to diffusion of theaniline monomer, the oxidant can diffuse through the membrane.Polyaniline nanofibers will form and then precipitate out of aniline andoxidant solution onto the substrate and may be subsequently collected.The polymer is deposited on the substrate as aligned nanofibers havingdiameters ranging from 5 nm to 250 nm or more, more preferably 5 to 60nm.

Suitable substrates include non-conductive and/or conductive substrates.Preferred non-conductive substrates include e.g., poly(ethyleneterephthalate) (PET), poly(methyl methacrylate) (PMMA), polystyrene(PS), poly(dimethyl siloxane) (PDMS), Teflon®, Tyvek®, Kapton® polyimidefilm, polyimide (PI), Polyethylene (PE), Polyurethane (PU), filterpaper, quartz, and glass. Preferred conductive substrates include e.g.,ITO, Pt, Au, Au coated glass, Stainless steel and Si wafer.

The permeable membrane may be any membrane through which the anilinemonomer and/or oxidant can diffuse or otherwise pass through. Thus,various types of cellulose or other finely porous materials may be usedas the membrane. Useful membranes may thus be made from, for example,regenerated cellulose, cellulose ester, or polyvinyldiene difluoride.The arrangement of the membrane can vary depending on the size, shape,etc. of the reaction chamber, with the only provision being that it mustseparate the monomer from the oxidant.

In one specific embodiment, applicants have found that conventionalregenerated cellulose dialysis tubing provides excellent results in thatit adequately controls the diffusion of monomer(s) or oxidant(s) toenable the production of extremely fine polyaniline nanofibers. Thus, inthis embodiment, aniline monomer in solution is placed in dialysistubing, which is then sealed. The sealed tubing is then placed in areaction chamber (such as a beaker) containing an oxidant in an acidsolution and a substrate onto which aligned polymer fibers will deposit.Alternately, the oxidant may be placed in the tubing with the anilinemonomer in the reaction chamber.

In this embodiment, the pore size of the dialysis tubing may be changedto control the rate of diffusion of the aniline and/or the oxidant andthus its concentration in the reaction chamber. This control can be usedto customize the size and configuration of the resulting polyanilinenanofibers, as described below. Applicants have found that a regeneratedcellulose membrane (or tubing) with a molecular weight cut off (MWCO) ofabout 3500 to 60,000 provides excellent results. Nevertheless, othermembranes with larger or smaller pore sizes may be used. Thus, celluloseester membranes with MWCO of from 100 to 300,000 or polyvinyldienedifluoride membranes with MWCO of from 250,000 to 1,000,000 are alsosuitable exemplary materials.

Polyaniline produced according to the process of this invention may beprepared from the polymerization of unsubstituted aniline or asubstituted aniline monomer. In addition, dimers as well as oligomershaving up to eight repeating aniline or substituted aniline units mayalso be used in the various embodiments. As used herein, any generaldescription using the terms “aniline” is intended to refer to andencompass both substituted and unsubstituted aniline monomer, as well asdimers or oligomers thereof of up to eight units in length. Likewise,the term “polyaniline” is also intended to refer to and encompasspolymers of both substituted and unsubstituted anilines, includingcopolymers thereof, unless specifically noted.

Exemplary substituted aniline monomers include those having thefollowing formula:

wherein, R₁, R₂, R₃, R₄, R₅, and R₆ are independently selected from thegroup consisting of: hydrogen atom, alkyl, alkoxy, alkylsulfonyl,arylsulfonyl, halogen, alkoxycarbonyl, alkylthio, alkylsulfuryl,cycloalkyl, sulfonic, aryl or carboxylic substituted alkyl substituents.

Specific substituted anilines that may be amenable to the presentprocesses include 2-aminobenzenesulfonic acid, 3-aminobenzenesulfonicacid, orthanilic acid, o-toluidine, m-toluidine, o-anisidine,m-anisidine, as well as polyhalogen anilines such as 2-fluoroaniline,2-chloroaniline, 2-bromoaniline, 2-iodoaniline, 3-fluoroaniline,3-chloroaniline, 3-bromoaniline, and 3-iodoaniline. In addition, it maybe possible to use other monomers by modifying the disclosed processesincluding, for example, pyrrole, substituted pyrrole, thiophene,substituted thiophene and 3,4-ethylenedioxythiophene as well as the useof two or more monomers to produce a copolymer, such as aniline/pyrrole,aniline/toluidine or aniline/anisidine. Specific nanofibers of bothpoly(-o-toluidine) and sulfonated polyaniline were successfully producedusing the present processes.

In an aqueous polymerization medium, any conventional protonic acid ormixtures thereof may be used in the present embodiments. Both inorganicand organic acids may be used including chiral acids. Such acids for usein the polymerization of aniline are known and include, but are notlimited to, protonic acids which can be used to form a complex with theaniline monomer and to make it possible for the aniline monomer to bedissolved in water. Exemplary acids include hydrochloric acid, hydrogenbromide, sulfuric acid, perchloric acid, nitric acid, phosphoric acid,phosphonic acid, trifluoromethanesulphonic acid, toluenesulphonic acid,dodecylbenzenesulphonic acid (DBSA), acetic acid, p-toluenesulfonic acid(p-TSA), trichloroacetic acid, trifluoroacetic acid, formic acid,(1R)-(−)-10-camphorsulfonic acid, (1S)-(+)-10-camphorsulfonic acid(CSA), 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPSA), andmethanesulfonic acid (CH₃SO₃H), carboxylic acids, etc. It is alsopossible to use a mixture of these protonic acids. Also Lewis acids canbe used. The invention is not limited to the use of the above-mentionedacids.

The oxidative agent used in the process according to the presentembodiments may be any conventional oxidizer used in the polymerizationof aniline. Exemplary oxidizing agents include ammonium peroxydisulfate(APS), persulfated salts such as, sodium persulfate and potassiumpersulfate, perchlorated salts such as potassium perchlorate,chlorinated salt such as potassium chlorinate, iodonated salt such aspotassium iodonate, chlorinated iron such as ferric chloride,halogenated metal acids such as chloroaurate acid, fuming sulfuric acid,and ozone. Preferred oxidants include APS, K₂Cr₂O₇, KlO₃, FeCl₃, KMnO₄,KBrO₃, KClO₃, peracetic acid or hydrogen peroxide. The reduced oxidantmay remain in the resulting polymer nanofibers, as for example, iron orgold nanoparticles.

The polymerization temperature in the process of the present embodimentsmay vary within a range from about −15 to 60° C. Similarly, the pH ofthe reaction solution is preferably maintained at a pH value of belowabout 1. However, nanofibers can also be produced at a pH value of 1 orabove.

As detailed above, the membrane is used to steadily control the releaseof aniline and/or oxidant into the other solution to form polyanilinenanostructures. In conventional bulk polymerization methods, the anilinemonomer is typically present in the reaction solution at a molarconcentration of about 0.3 to 0.6. In the specific embodiments hereinwherein the aniline diffuses through the membrane to react with theoxidant (thus making the oxidant solution the site of the reaction),there may be a much lower concentration of aniline in the reactionsolution, for example on the order of about 0.001 to about 0.01 M,preferably about 0.008 M. Alternately, if the oxidant is the speciesthat diffuses through the membrane (thereby making the aniline solutionthe site of reaction), then the concentration of oxidant in the reactionsolution may fall within the above ranges.

This low concentration is achieved in these embodiments by the slowdiffusion of aniline (or oxidant) across the membrane. Aniline can beused directly or dissolved in any acid solutions or in any organicsolvents with any concentrations. In the embodiment below, this lowconcentration is achieved by the introduction of much smaller amounts ofaniline into the reaction chamber. The amount of oxidant initiallypresent in solution prior to polymerization relative to the amount ofaniline initially present in solution is not critical, but applicantshave found that a preferred molar ratio of aniline to oxidant is 1:1. Ithas been found that stirring or otherwise agitating the reaction mixtureduring polymerization may be desirable in some instances to producenanofibers having specific characteristics.

In the first embodiment the characteristics of the resulting polymernanofibers (including diameter and morphology) can be controlled to acertain degree by the selection of acid to be added to the reactionmixture as well as the temperature at which the polymerization iscarried out and the inclusion of a surfactant.

In the first embodiment suitable surfactants that may be used in thereaction system include anion surfactants such as sodium dodecylsulfate,cation surfactants such as dodecyltrimethylammoniumbromide, and nonionicsurfactants such as Triton® X-100. When included, the concentration ofsurfactant in the reaction mixture may range from, e.g., 0.0001 M to 1M.

The resulting doped polymer can be dedoped with a base to produce anon-electrically conductive polyaniline product (emeraldine base) withelectrical conductivity less then 10⁻⁵ S/cm, which can be re-doped witha suitable acid to produce an electroconductive polymer with the desiredproperties. By this dedoping and redoping process, it is possible tocontrol the electro-conductive properties of the polymer nanofiberscontinuously over the full range from that of an electrical insulator(conductivity <10⁻¹⁰ S/cm) to that of a semiconductor (conductivity˜10⁻⁵ S/cm) to that of a good conductor of electricity (conductivity ˜1S/cm) to that of a metal (conductivity >10² S/cm).

In a second embodiment, the “dilute polymerization process” describedabove, bulk polymerization of aniline or substituted aniline monomer isconducted at very low concentration of aniline monomer. This isaccomplished by introducing aniline monomer solution into an oxidantsolution (or vice versa) and polymerizing at very low concentrations.Suitable concentrations may be tens of millimoles or lower andpreferably from 0.001 to 0.01 M. Applicants have found that this lowconcentration allows the production of polyaniline nanofibers.Applicants have found that this low concentration coupled with theeffect of minimal or not stirring or agitating the reaction mixtureduring the polymerization, allows the production of polyanilinenanofibers of increased length. However, it has been found that stirringor otherwise agitating the reaction mixture during polymerization may bedesirable in some instances to produce nanofibers having specificcharacteristics.

In this second embodiment, a typical bulk polymerization reactionapparatus may be used. This typically consists of a reaction chamber,which in its simplest form may be a beaker. An aqueous solution ofprotonic acid, oxidant(s) and, if necessary, other agents are added intothe reaction chamber. A substrate is included in the reaction containeronto which aligned polymer fibers are deposited. Oxidant(s) can bedissolved in an acid solution for example in the mixing tank. A commonlyused oxidant is ammonium peroxydisulfate (APS). Also other oxidants canbe used. Protonic acid makes the polymerizing medium acidic, therebymaking the polymerization reaction possible. Protonic acid also acts asa so-called dopant which donates the counter anion and forms a salt withthe polyaniline base. Suitable acids are described above.

The actual polymerization takes place by feeding monomer(s), e.g.aniline into the process. Dissolved into a suitable medium, such as anaqueous acid solution, aniline is supplied to the reaction chamber.Depending on the temperature of the reaction mixture, the polymerizationtakes place over the course of several hours. While stirring istypically used in the polymerization of aniline and can be performed inthe present process, applicants have found that longer and less branchedfibers are possible if the mixture is subjected to minimal stirring orotherwise not agitated. Polymerized aniline precipitates and isdeposited on the substrate, which can then be collected and purified.

The amount of oxidant initially present in the reaction solution priorto polymerization relative to the amount of aniline initially present inthe reaction solution is not critical, with the initial molar ratio ofaniline to oxidant ranging from 50:1 or greater down to 0.02:1. Morepreferred molar ratios are from 10:1 to 0.1:1 and even 4:1 to 1:1. Aparticularly suitable molar ratio of aniline to oxidant is 1.5:1.

In the second embodiment the characteristics of the resulting polymernanofibers (including diameter and morphology) can be controlled to acertain degree by the selection of acid to be added to the reactionmixture as well as the temperature at which the polymerization iscarried out and the inclusion of a surfactant.

In the second embodiment suitable surfactants that may be used in thereaction system include anion surfactants such as sodium dodecylsulfate,cation surfactants such as cetyltrimethylammoniumbromide, and nonionicsurfactants such as Triton® X-100. When included, the concentration ofsurfactant in the reaction mixture may range from, e.g., 0.0001M to 1 M.

Functionalized polyaniline can likewise be produced by modifying themolecular structure of polyaniline after polymerization without changingthe nanostructure of the aligned fibers. Thus, nucleophilic addition ofthe sulfite ion (—SO₃ ⁻) to the polyaniline fibers can be made by addingfuming sulfuric acid to the polymer via the following reaction:

FIG. 1 shows the nanostructure of such fibers both before and afternucleophilic addition.

Aligned and oriented copolymers, e.g. sulfonanted polyaniline, can alsobe synthesized via our proposed two methods. FIG. 2 shows SEM images forsulfonanted polyaniline synthesized from different concentration of themonomers (aniline and 3-aminobenzenesulfonic acid). The density ofaligned nanofibers deposited on the substrate can be controlled throughthe concentration of monomers.

The polymer nanofiber networks so made can be used for chemical,biological, environmental or medical sensors as well as the activechannel of a field effect device.

The following examples are provided for purposes of describing thepreferred embodiments. They should not be considered limiting of theinvention.

EXPERIMENTAL

Reagents

Aniline (Aldrich) and pyrrole (Aldrich) were distilled under vacuumbefore use. Ammonium peroxydisulfate (APS; 99.99%, Aldrich), deionizedwater, and dopant acids were used directly as received without furtherpurification. Spectra/Por Dialysis Tubing, Regenerated Cellulose (MWCO3500, MWCO 12k-14k, MWCO 15K, MWCO 25K, and MWCO 60K) and Spectra/PorClosures were purchased from Spectrum Laboratories, Inc.

A variety of inorganic/organic dopant acids were used to study theformation of polyaniline nanofibers in the dilute polymerization andporous membrane controlled polymerization, including hydrochloric acid(HCl), sulfuric acid (H₂SO₄), perchloric acid (HClO₄), phosphoric acid(H₃PO₄), 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPSA),p-toluenesulfonic acid (p-TSA), (1S)-(+)-10-camphorsulfonic acid (CSA),methanesulfonic acid (CH₃SO₃H), nitric acid (HNO₃), etc.

Substrates included Si-wafer, PET film and ITO coated glass.Polyethylene terephthalate over-head transparency film (JPET; Universityof Yamanashi, Collage Bookstore, Japan) were used with/without acetonetreatment. Tissue paper with acetone was used to remove the surfacecoating of JPET by gentle wiping in one direction. A glass coated withITO and other types of substrates were cleaned by sonication in RBSsolution (10 ml of concentrated RBS-38, 5 ml of ethanol and 485 ml ofdeionized water) for at least 30 minutes and then rinsed by a plenty ofdeionized water before use.

Example 1 Dilute Polymerization Trials

Aniline was dissolved in predetermined amounts in 1M dopant acidsolution, and carefully transferred to a solution of ammoniumperoxydisulfate (APS) dissolved in 1M dopant acid solution in beakers.The reaction was carried out at various temperatures (room temperature,0-5° C. in an ice bath, 12-14° C. in a freezer) both with and withoutdisturbance (e.g. stirring). After 24 hours, the dark-green polyanilineprecipitate was collected either by dialysis tubing (MWCO 12k-14k), andthen purified by dialysis with deionized water or using a Buchner funnelwith a water aspirator and then purifying portionwise with deionizedwater until the filtrate became colorless. Trials were conducted usingvarious concentrations of aniline to study the formation of polyanilinenanofibers, ranging from 8 mM to 128 mM. The molar ratio of aniline andAPS was also varied from 4.35:1 to 1:1. A variety of acids were used asdetailed above. 0.2 M to 1.2 M of dopant acids were used to studyformation of nanofibers.

Example 2 Porous Membrane Controlled Polymerization Trials

regenerated cellulose dialysis tubing with molecular weight cut-off(MWCO) 3,500 (Spectra/Por 3, Spectrum Laboratories, Inc.) was used as apermeable membrane, aniline as a monomer, ammonium persulfate as anoxidant and various inorganic/organic acids as dopant acids. Aniline wasdissolved in 12 mL of 1M dopant acid solution, and carefully transferredto dialysis tubing (MWCO 3500) sealed with Spectra/Por Closures. Thesealed dialysis tubing was put into a 600 mL beaker with a solution ofammonium peroxydisulfate dissolved in 500 mL of 1M dopant acid solution.The reaction was carried out at room temperature and at a temperature0-5° C. (ice bath) both without any disturbance as well as withstirring. After 24 hours, the precipitated dark-green polyanilineoutside the dialysis tubing was collected and purified using the sameprocedures as mentioned in the dilute polymerization. Trials wereconducted using various concentrations of aniline to study the formationof polyaniline nanofibers, ranging from 8 mM to 128 mM.

As an illustrative case, we chose regenerated cellulose dialysis tubingwith molecular weight cut-off (MWCO) 3,500 (Spectra/Por 3, SpectrumLaboratories, Inc.) as a permeable membrane, aniline (Aldrich) distilledunder vacuum as a monomer, ammonium persulfate ((NH₄)₂S₂O₈, APS; 99.99%,Aldrich) as an oxidant and perchloric acid (HClO₄; 70%, GFS Chemicals)as a dopant acid. Aniline (4.025 mmole) was dissolved in 12 mL of 1MHClO₄ solution, and carefully transferred to dialysis tubing (MWCO 3500)sealed with Spectra/Por Closures. The sealed dialysis tubing was putinto the 600 mL beaker with the solution of ammonium peroxydisulfate(4.025 mmole) dissolved in 500 mL of 1M HClO₄ solution. The reaction wascarried out at temperature 0-5° C. (ice bath) without any disturbance.After 24 hours, the precipitated dark-green polyaniline outside thedialysis tubing was collected and purified by dialysis against deionizedwater (Dialysis Tubing, MWCO 12k-14k).

In both the Dilute Polymerization trials and the Porous MembraneControlled Polymerization Trials, the doped polyaniline nanofibers orsamples prepared here were dedoped by dialysis with 0.1 M NH₄OH_((aq)),and then redoped by dialysis with 0.5M or 1M HCl_((aq)). It is notedthat any kind of acid as mentioned above can be used to dope or redopenanofibers and a basic solution (e.g. NH₄OH_((aq)), NaOH, etc.) can beused to dedope the nanofibers.

Characterization and Results

Samples deposited onto Si-wafer substrates and then sputtered with athin layer of Au/Pd or Au were used for scanning electron microscopy(SEM, Philips XL-30 ESEM) studies. Samples dispersed in deionized waterwere transferred to copper grids for transmission electron microscopy(TEM, Philips CM-200 or Philips TF-20). UV/vis absorption was studiedusing a UV/VIS/NIR Spectrometer (PERKIN ELMER Lambda 19) employing thepolyaniline dispersions in deionized water. Samples dispersed indeionized water were transferred to Rigaku glass powder holder for thestudy of X-ray diffraction. X-ray patterns were taken with CuKαradiation (λ=1.52 Å).

Dilute Polymerization

Typical TEM and SEM images obtained demonstrate the nanofibrousstructures of polyaniline as shown in FIG. 3 and FIG. 4, respectively.As long as polymerizations are performed using dilute aniline,nanofibers are formed regardless of which acids are used, includingthose mentioned in the experimental section. Polyaniline nanofibersproduced with different acids show similar nanofiber structures ofinterconnected networks. However, different types of acids can producepolyaniline nanofibers with different diameters. As shown in FIG. 3( a),polyaniline nanofibers obtained from CSA_((aq)) present smallerdiameters of 17-50 nm. The diameters of polyaniline nanofiberssynthesized in CH₃SO₃H_((aq)) range from 42 nm to 70 nm as shown in FIG.3( b). In addition, FIG. 3( c) shows larger diameters of polyanilinenanofibers obtained from HClO_((aq)) varying from 72 nm to 250 nm. Thediameters of nanofibers described here are based on multiple statisticalTEM measurements on different areas. Nanofiber morphology for each ofthe acids used is reproducible for the same synthesis conditions, suchas concentration, temperature, etc. This indicates that the diameters ofpolyaniline nanofibers can be controlled directly by the appropriateselection of dopant acids. In addition, nanofibers obtained fromdifferent dopant acids all show similar range of lengths up to severalmicrometers.

The morphology of doped nanofibers does not significant change whendedoped and redoped multiple times by 0.1M NH₄OH_((aq)) and 1M dopantacids, respectively. However, the nanofibrous structures are deformed orbroken into fragments of small pieces under high-pressure mechanicalstress, e.g. 2500 KPa, or an intense ultrasonic bath. Polyanilinenanofibers obtained in dilute aniline are easily dispersed in deionizedwater, methanol, ethanol or toluene etc., and the resulting suspensionsare stable for several minutes, followed by agglomeration andprecipitation. The suspension increases the processibility ofpolyaniline nanofibers. For example, one can use the suspension ofnanofibers to cast a thick film, ˜10 μm, on a substrate and then immerseit into electrolyte solutions or solvents. Surprisingly, the thick filmis stable in the solutions or solvents for more than one day without anydissolution. This may reflect the strong interaction and the highlyentangled aggregation among the nanofiber networks as shown in SEMimages (FIG. 4).

FIG. 5 shows a scanning electron micrograph (SEM) of polyanilinenanofibers obtained in 1M HCl_((aq)), wherein (a)-(b) show fibersproduced via conventional polymerization wherein [aniline]=0.4M, and(c)-(d) show fibers produced via dilute polymerization wherein[aniline]=0.008M. The white spots in the dashed circle in (d) maycorrespond to surface active sites for nucleation.

The formation mechanism of polyaniline nanofibers remains unclear. It issuggested that the growth of nanofibers is intrinsic to thepolymerization of aniline because it is observed even during synthesiswith a relatively high concentration of aniline (SEM image, FIG. 5(a)-5(b)). However, in contrast to polymerization in a dilute solution(FIG. 5( c)-5(d)), for polymerization in concentrated solution theindividual polyaniline nanofibers pack very densely and merge with eachother as shown in FIG. 5( b). According to classical nucleation theory,the nanofibers formed initially may serve as nucleation sites foradditional nanofibers (secondary nucleation). We observe small whitespots on the nanofiber surfaces, e.g. FIG. 5( d) within the dashedcircle, which are associated with asperities along the fibers. These mayact as surface active sites for nucleation.

Although not intended to be bound by any theory, it is believed that theformation of interconnected, branched nanofiber networks may beexplained as follows: In the event of high aniline concentration insolution, a competition between directional fiber growth process andformation of additional nucleation centers is taking place. Once a highdensity of nucleation centers is generated, the interfacial energybetween the reaction solution and nanofibers may be minimized and hencerapid precipitation occurs, in a disordered manner, yielding irregularshapes. In a dilute solution the number of nucleation sites formed onthe surface of the nanofibers may be reduced, thus allowing polyanilineto grow only in a one-dimensional morphology.

Polyaniline/HClO₄ nanofibers are dispersed in deionized water viavigorously shaking by hand for UV/vis absorption studies. FIG. 6 showsUV/vis absorption spectra of polyaniline nanofibers obtained fromHClO_(4(aq)). After purification by dialysis against deionized water,polyaniline nanofibers present three absorption peaks c.a. 338 nm, 430nm and 960 nm (free carrier tail) as shown in the dotted line of FIG. 6.By adding one drop of dilute 70% w/w HClO_(4(aq)) to the abovepolyaniline nanofiber dispersion, the absorption intensity of the peaksat c.a. 338 nm decreases, simultaneous with the increase of theabsorption intensity of the peak at c.a. 430 nm and 960 nm (free carriertail) as shown in the dashed line of FIG. 6.

It is possible that the purification of polyaniline nanofibers bydialysis with deionized water results in removal of the dopant, HClO₄,within polyaniline backbone to form partially doped polyanilinenanofibers. Furthermore, adding a drop of 30% w/w NH₄OH_((aq)) topolyaniline/HClO₄ nanofibers dispersion introduces the formation of anabsorption band c.a. 677 nm, simultaneously resulting in disappearanceof two absorption bands c.a. 430 nm and 960 nm (free carrier tail) asshown in the solid line of FIG. 6. The two strong absorption bands c.a.338 nm and 677 nm are attributed to the formation of emeraldine base.

FIG. 7 shows X-ray diffraction (XRD) pattern of doped polyanilinenanofibers obtained from HClO_(4(aq)). Two intense broad bands centeredat 2θ˜20° and ˜25° show that these nanofibers are partially crystalline.Comparing to previously reported data, there are no significantdifferences between the structural order of polyaniline nanofibers andthat of nonfibrous polyaniline powders or films.

The suspension of polyaniline nanofibers obtained from HClO_(4(aq)) wasdeposited and dried to form a dark green film on a glass slideprepatterned with four Au electrodes constructed by thermal evaporation.The bulk room temperature DC conductivities of polyaniline/HClO₄nanofibrous film are in the range of 2-4 S/cm obtained from the standard4-probe method. Based on UV/vis studies as shown in FIG. 6, we suggestthat an individual nanofiber has a higher room temperature DCconductivity than nanofibrous films. This is supported by the freecarrier tail associated with delocalization of electrons in the“polaron” band. As the inter-fiber interaction is negligible in thesevery dilute suspensions, the characteristics of the spectra are relatedto the properties of the individual nanofibers only. Furthermore, asshown earlier, there also is significant inter-fiber contact resistancebetween the individual nanofibers produced by using surfactants, therebydecreasing of the bulk conductivity.

In summary, polyaniline nanofibers are successfully synthesized for thefirst time using dilute polymerization resulting in the careful controlof nucleation and growth. This is accomplished via reducing theconcentrations of both monomer and oxidant with a constant molar ratio.We also demonstrate that different dopant acids produce similarmorphology of polyaniline nanofibers. Additionally, the diameters ofpolyaniline nanofibers are tunable under the appropriate selection ofdopant acids. The dispersion of polyaniline nanofibers can be cast toform highly porous nanofibrous films without deformation of thenanofiber morphology.

Porous Membrane Controlled Polymerization

A variety of inorganic/organic dopant acids were used to study theformation of polyaniline nanofibers in the porous membrane controlledpolymerization, including hydrochloric acid (HCl), sulfuric acid(H₂SO₄), phosphoric acid (H₃PO₄), perchloric acid (HClO₄),2-acrylamido-2-methyl-1-propanesulfonic acid (AMPSA), p-toluenesulfonicacid (p-TSA), (1S)-(+)-10-camphorsulfonic acid (CSA), methanesulfonicacid (CH₃SO₃H), etc. All precipitates show very similar nanostructuredmorphology. In addition, regardless of molar ratios of aniline to APSused, e.g. ranging from 4/1 to 1/2, and the concentration of dopantacids used, e.g. 0.2M to 6M, polyaniline precipitates obtained allpresent nanofibrous structures. However, the average diameters ofnanofibers are strongly dependent on polymerization temperature. Forexample, nanofibers synthesized in HClO_(4(aq)) at room temperature showlarger diameters ranging from 70 nm to 180 nm (average diameter ˜130 nm)confirmed via TEM compared to those obtained at lower temperature, 0˜5°C. (ranging from 50 nm to 100 nm; average diameter ˜80 nm). It isobvious that temperature controls the releasing rate of the reagentsthrough the porous membrane.

As shown in FIG. 8 (SEM and TEM images), the dark green precipitate iscomprised of nanofibrous structures with diameters ranging from 50 nm to100 nm (average diameter ˜80 nm), confirmed via TEM. The uniformity ofpolyaniline nanofibers obtained can be observed from the SEM image ofFIG. 8A at low magnification (×100), with a scale bar=200 μm. It showsnearly ˜100% of nanofibers formed in this chemical polymerization withthe porous membrane control, which is demonstrated by a highmagnification SEM image (×20,000) of FIG. 8B. FIG. 8 also show thatpolyaniline nanofibers are of the interconnected, branched and networkedmorphology. However, after dilution of the colloid suspension ofpolyaniline nanofibers with large amounts of water or ethanol, somesingle polyaniline nanofibers can be isolated from the agglomeration(FIG. 8C). It indicates that this kind of polyaniline nanofibers can bepotentially used to fabricate nanoelectronic devices such asfield-effect devices. Polyaniline nanofibers formed within the dialysistubing also show very similar nanostructures as these found outside.There are also no significant differences on the nanostructuredmorphology as nanofibers are dedoped and redoped multiple times by baseand acid solutions, respectively.

FIG. 9 shows XRD patterns of polyaniline nanofibers obtained. Afterpurification by deionized water, as-prepared polyaniline nanofiberswhich is in-situ doped by HClO₄ are dispersed in 0.1M HClO_(4(aq)),termed ES-ClO₄. The XRD pattern of ES-ClO₄ reflects a partiallycrystalline structure (FIG. 7A). ES-ClO₄ belongs to class I (ES-I) andis typical for partially protonation with HClO_(4(aq)). One intensebroad band centered at 2θ˜20°, a shoulder at 2θ˜30°, and a weak band at2θ˜45° show that emeraldine base form (EB) of polyaniline nanofibersobtained is a strongly disordered polymer (FIG. 9B) which agrees withthe electron diffraction pattern obtained (not shown). This diffractionpattern is in agreement with that of non-fibrous emeraldine basesamples, EB-I. When nanofibrous EB is redoped by HCl_((v)) vapor(labeled as ES-Cl-vapor) or aqueous 0.5M HCl_((aq)) (labeled asES-Cl-p), new crystalline structures are introduced (FIGS. 9, C and D).Comparing to our earlier publications, the XRD patterns of ES-Cl-vaporand ES-Cl-p are ascribed to class I material (ES-I) by the preparationprocedures, and generally possess ES-I crystal structures. Similarly,the X-ray diffraction patterns of ES-Cl-vapor (FIG. 9C) and ES-Cl-p(FIG. 9D) are similar to that of ES-I as expected.

UV/vis absorption spectra (FIG. 10) demonstrate that polyanilinenanofibers obtained have different absorption patterns corresponding todifferent protonation levels and are in excellent agreement with theUV/vis absorption spectra of nonfibrous polyaniline powders of thesimilar doping level. This supports the presence of the same chemicalstructure for nanofibrous polyaniline and granular (nonfibrous)polyaniline. Furthermore, we note that based on UV/vis absorption (FIGS.10, B and D), ES-Cl-p shows partial protonation with HCl.

The dispersion of polyaniline nanofibers was deposited onto a glassslide to form a dark porous green film. Subsequently, four Au electrodeswere deposited by thermal evaporation to form the contacts. The bulkconductivity for the redoped nanofiber (ES-Cl-p) array is obtained inthe range of 0.9-1.3 S/cm based on the standard 4-probe DC measurementat room temperature. As the sample ES-Cl-p is exposed to HCl_((v))vapor, the conductivity is changed to 2.3-2.5 S/cm. This reconfirms thatES-Cl-p is partially doped by HCl as mentioned above. The bulk roomtemperature DC conductivity of the nanofiber film is comparable to thatof the nonfibrous powder. However, we suggest that an individualnanofiber has a higher conductivity than those films because the freecarrier tails associated with delocalization of electrons in the“polaron” band are shown in UV/vis absorption spectra of dopedpolyaniline nanofibers (FIG. 10). As very dilute dispersion ofnanofibers is used in the optical study, the characteristic of UV/visabsorption is only related to the properties of individual nanofibers.Thus, the lower bulk R.T. DC conductivity of the nanofiber film isattributed to the existence of the inter-fiber contact resistant.

For increased control of the nanofiber preparation, we used two segmentsof dialysis tubing to separate aniline and oxidant, respectively. Theproduct shows extremely similar nanofibrous morphology to those obtainedfrom one dialysis tubing used. When the solvent inside dialysis tubingis replaced with organic solvents, e.g. toluene, no obvious differenceis observed for the nanofibrous morphology. The nanofibers is alsoformed exclusively even when reaction is carried out with disturbance(e.g. stirring), but the most products are preferred to precipitateinside the tubing. Hence, the approach described here provides asignificant possibility to synthesize conductive/semiconductive polymerand copolymer nanofibers. We note that it may also be widely applicableto the fabrication of the nanofibers of non-conductive polymers.

In summary, high-quality polyaniline nanofibers are successfullysynthesized for the first time using porous membrane to control thepolymerization of polyaniline resulting in the careful control ofnucleation and growth. The diameters of polyaniline nanofibers aretunable under the appropriate selection of dopant acids and/ortemperature. The dispersion of polyaniline nanofibers can be cast toform highly porous nanofibrous films without deformation of thenanofiber morphology. UV/vis absorption and X-ray diffraction structureshow that polyaniline nanofibers have similar absorption and diffractionpatterns to those previously reported for nonfibrous polyaniline.

Morphology, Alignment and Orientation of Fibers

The highly uniform, green and transparent thin film deposited by dilutepolymerization or porous membrane controlled polymerization is verystrongly adhered onto most any substrate. The morphology of the thinfilm is examined by SEM. As shown in FIG. 11, the thin film deposited bydilute polymerization shows morphology with aligned and well-orientedpolyaniline nanofibers. The thin film shows the highly uniform whitebright spots when the film is examined from top view (FIG. 11( a)-(b)).Those whit spots actually are the tips of nanofibers (FIG. 11( c)). Whenthe sample is tilted a certain angle (e.g. an angle 50°), thosenanofibers appear (FIG. 11( d)-(f)). A close view to the edge of partlyremoved thin film shows that those nanofibers are perpendicular to thesubstrates (FIG. 11( f)). The diameters of the tips of the singlenanofibers are ranging from 20 nm to 40 nm. Some nanofibers packtogether to form a bundle resulting in a larger white spot withdiameters ranging from 40 nm to 70 nm. The average length of the uniformaligned nanofibers is ˜360 nm, confirmed by Surface Profiler (TencorInstruments, Alpha-Step 500).

The morphology of the aligned and oriented nanofibers is sensitive tothe temperature and concentration of the reagents used forpolymerization. For example, low temperature advantages the formation ofaligned and oriented nanofibers. The morphology of the deposited thinfilm is extremely similar to those nanofibers obtained in the bulksolution when the reaction is carried out at the room temperature (e.g.21˜24° C.).

When the initial concentration of aniline is used in the range of 8 mMto 12 mM and reaction is performed at low temperature (e.g. 0˜5° C.),the final morphology of the thin film is aligned and oriented nanofibersof perpendicularity to the substrates (FIG. 12). There is no apparentdifference on the morphology of aligned and oriented polyanilinenanofibers when the sizes of the reaction chambers or containers(reaction volume) are reduced or enlarged as long as the initialconcentration of the reagents is fixed in the range of 8 mM to 12 mM.All morphology of aligned and oriented polyaniline nanofibers for thesame polymerization conditions is reproducible. This indicates that themethods we propose here allow us to scale up the coating on any types ofthe substrates.

We have successfully deposited the aligned and oriented nanofibers ontoany size and any geometry substrate, including submicron PET fibers, anysize of the micron channels and micron pillars, 8.5 inches×11 inchesletter sized transparency, any size of ITO coated glass, etc. The samemorphology is also observed on molecularly modified surfaces of thesubstrates (e.g. 3-aminopropyltriethoxysilane (APTES)-modified glass).The free-standing thin film collected at the interface of air andpolymerization solution also shows the same morphology as those found onthe substrates. Disturbance (e.g. stirring) of the reaction and dopantacids are only slightly effect on the final morphology. However, moreuniform coating is produced when the monomer and oxidant are distributedhomogenously during the polymerization.

In one trial, aligned and oriented polyaniline nanofibers are depositedonto over-head PET transparency for UV/vis absorption studies. Thesurface coating of the over-head PET transparency is gently wiped awayin one direction using tissue paper with acetone. FIG. 13 shows UV/visabsorption spectra of the thin film polyaniline obtained fromHClO_(4(aq)). Two absorption bands c.a. 430 nm and 860 nm (free carriertail) are observed for the different initial concentration of anilineused for polymerization. The UV/vis absorption patterns of the thin filmof aligned and oriented polyaniline nanofibers obtained are consistentwith previously reported results. It is obvious that the absorptionintensities at 400 nm vary from the initial concentration of anilineused for polymerization. As shown in FIG. 14, the maximum UV/visabsorption intensity occurs for the 10 mM (0.01 M) of the initialconcentration of aniline. The thickness of the thin film is also foundto be varied by the initial concentration of aniline. For example, themeasured thickness of the thin film is ˜360 nm for 10 mM of aniline and˜300 nm for 12 mM of aniline. Thus, the maximum UV/vis absorptionintensity at 400 nm may be correlated to thickest thin film formed withthe morphology of uniform aligned and oriented polyaniline nanofibersonly if the initial concentration of aniline is fixed at 0.01M.

The coating surfaces of aligned polyaniline nanofibers aresuperhydrophilic, but can be changed to become superhydrophobic afterexposure to CF₄ plasma. Being able to modify the molecular structure ofpolyaniline nanofibers without damaging the nanostructures broadens theapplicability of polyaniline nanofibers, as described more fully below.

APPLICATIONS

Surface Response

Superhydrophilic/Superhydrophobic and Superoleophilic/OleophobicSurfaces

Surface modification has attracted much attention in the field ofnanotechnology in the recent years. Wettability of solid surfaces isgoverned by both surface energy and roughness or surface structures. Thesurface energy is the intrinsic property of each solid materialdependent on the chemical composition. In general, a hydrophobicsurface, in which the water contact angle is enhanced by small roughnessand is higher than about 150°, is called “superhydrophobic”, and ahydrophilic surface, in which the water contact angle is similarlyreduced by small roughness and is less than 5°, is called“superhydrophilic.” Similarly, an oleophobic surface, in which thecontact angle of an oil on the surface is enhanced by small roughnessand is higher than about 150°, is called “superoleophobic”, and aoleophilic surface, in which the oil contact angle is similarly reducedby small roughness and is less than 5°, is called “superoleophilic.” Anoleophobic surface, with a contact angle of between 90 and 150° issimply considered “oleophobic”.

Those phenomena can be explained by Wensel equation Eq. (1):cos θ=r(γ_(SL)−γ_(SL))/γ_(LV)  (Eq. (1))

where γ_(SV), γ_(SL) and γ_(LV) are the interfacial energies per unitarea of the solid-gas, solid-liquid and liquid-gas, respectively, and ris a roughness factor, which is defined as the ratio of the actual areaof the rough surface to the geometric projected area and is alwayslarger than 1. Therefore, from equation (1), we know that the roughnessof the substrates can enhance both the hydrophobicity of the hydrophobicsurfaces and the hydrophilicity of the hydrophilic ones, as shown inFIG. 15. In addition, subsequent researchers assumed that the air issuperhydrophobic with a water contact angle of 180°. They claimed thatair trapped on the surfaces may enhance the hydrophobicity. Thus, theyproposed equation (2) to describe the contact angle at a solid surfacein the presence of air.cos θ=f cos θ+(1−f)cos 180°=f cos θ+f−1  (Eq. (2))

where f is described as the fraction of a wetted solid surface area toactual area of the solid surface and air is assumed to besuperhydrophobic with a water contact angle of 180°.

Aligned and Oriented Polyaniline Nanofibers

The surfaces coated by aligned polyaniline nanofibers show the propertyof superhydrophilicity with water contact angle less than 5 degrees.However, after exposure to a fluorocarbon plasma (such as CF₄ or CHF₃plasma), these same surfaces exhibit a dramatic change tosuperhydrophobicity and water contact angles higher than 175°. Theplasma thus lowers the surface energy. Such novel coatings provide awider route to synthesize nanostructures of conductive, semiconductive,and non-conductive polymers for use in controlling the delivery of DNAvia nanoneedle patches.

Dual Surface Response

Directly Patterned Surfaces with Superhydrophilicity/SuperydrophobicitySuperoleophilicity/Oleophobicity

One can use a nucleophile (e.g. sodium metabisulfite, sodium bisulfite,sodium sulfite, or sulfite salts) to modify the surfaces of polyanilinenanofibers in base form. After molecular modification, the surface stillmaintains the superhydrophilicity because the functional group ofsulfite (—SO₃ ⁻) covalently bonded to the backbone of polyanilinenanofibers has higher surface energy (i.e. hydrophilic). Thus, when theentire surfaces are exposed to low surface energy materials in acid form(e.g. DBSA, fluorinated alkyl sulfonic acid, etc.), the modified area isnot redoped by such acids. This leaves the modified areasuperhydrophilic and causes the unmodified area to become hydrophobic.Thus, we can create patterns on the surfaces of polyaniline withdifferent properties. This technique may be applied in order to writepatterns on the surface of aligned and oriented fibers through, e.g. anink-jet printer.

Multilayer Nanostructures

Polyaniline has three different oxidation/reduction forms. Generally,standard chemical synthesis produces emeraldine salt. After the dedopingprocess by basic solution, emeraldine salt (ES) turns to emeraldine base(EB). When EB form is exposed to oxidizers, it turns to pernigranilinebase (PB). The color of the aligned nanofibers coated film changes fromblue to pink. The oxidation potential of pernigraniline oxidation stateis ˜0.8 V vs. SCE, which is sufficient to polymerize (initiate) somemonomers (e.g. pyrrole (0.5 V vs SCE). Thus, there is contemplated theprocess wherein a conducting or non-conducting polymer is polymerized onthe oriented nanofibers. This process can be used to create differentsurface properties. In one exemplary process, pyrrole in 1M HCl isdeposited on the aligned nanofibers. The pyrrole is then polymerized,forming aligned fibers as well. After deposition and polymerization ofthe pyrrole, the average size of the tips changes to 120 nm. Otherexemplary polymers that can be used includepoly(3,4-ethylenedioxythiophene), polystyrene andpoly(2,3,4,5,6-pentafluorostyrene). The polymer can polymerize to form aconformal coating on the aligned nanofibers. This process opens apossible route to synthesize the oriented nanostructures of otherconductive polymers. These multilayer-type oriented nanostructures willhave broad applicability in many areas.

Non-conductive polymer, e.g., polystyrene orpoly(2,3,4,5,6-pentafluorostyrene), can be grafted onto the surfaces ofaligned polyaniline nanofibers. The grafted polymerization may beperformed by exposing styrene or 2,3,4,5,6-pentafluorostyrene, which arecast on top of aligned polyaniline, to UV irradiation at ˜50-70° C. fora period of time under nitrogen. Styrene or 2,3,4,5,6-pentafluorostyrenealso can be initiated and then polymerized onto aligned polyanilinenanofiber surface at ˜50-70° C. for a period of time under nitrogenusing the free-radical initiator (e.g., azobisisobutyronitrile (AIBN),benzoyl peroxide, or 1,1-azobis(cyclohexanecarbonitrile)). Non-graftedhomopolymers of polystyrene and poly(2,3,4,5,6-pentafluorostyrene) canbe removed by chloroform, toluene, xylene or tetrahydrofuran. The amountof styrene or 2,3,4,5,6-pentafluorostyrene used can be varied from 100%to 1% w/w (diluted by chloroform, toluene, xylene or tetrahydrofuran).

Pattern by Printer Toner (Line-Patterning)

An easy patterning method, i.e. line patterning, is used to demonstratethat our two methods as mentioned above to deposit aligned polyanilinenanofibers can also be grown on pre-patterned substrates. The patterncan be constructed using any removable material on a substrate. Althoughnot intended to be limiting, the removable material can be, e.g., an inkor toner printed using any conventional type of printer on a substrateto which it loosely adheres (such as an over-head PET transparencyfilm). The aligned fibers are then deposited on the substrate, such thatboth the pattern and at least a part of the remainder of the substrateis coated. The removable material is then removed by known methods (suchas sonication in the case of ink of an over-head film). This removes thealigned nanofibers covering the pattern as well, while keeping thealigned nanofibers on the rest of the substrate intact, creating, ineffect, a negative pattern of aligned nanofibers.

Thus, in one example, a line pattern was constructed by MicrosoftPowerPoint 2000 SR-1 and printed by Hewlett-Packard LaserJet 4M on anover-head transparency film. As shown in FIG. 16, the coated area showsthe morphology of aligned polyaniline nanofibers. After the removal ofprinter toner, there is no apparent difference in the morphology ofaligned nanofibers. This indicates that we may use this method toconfine the growth of aligned polyaniline nanofibers on the certain areafor the applications on chemical sensors, biosensors, etc.

DNA Stretching on the Surfaces of Aligned and Oriented Nanofibers

We use the coated surfaces of aligned polyaniline nanofibers prepared byabove two methods to stretch DNA. After the surfaces are coated byaligned polyaniline nanofibers, they show superhydrophilicity. Thisindicates that a water droplet will spread out on the surfaces with acontact angle less than 5°. When the aqueous solution of λ-DNA isinjected to the coated surfaces, DNA is stretched by the strongcapillary force resulting from the superhydrophilic coated surfaces. Thesame effect is seen if polypyrrole is deposited on a surface of thealigned polyaniline.

Low Work Function Electrodes Fabricated by Aligned and OrientedNanofibers for Organic Light-Emitting Diodes

We also demonstrate that the surface coating of aligned polyanilinenanofibers can be used as the electrodes for organic light-emittingdiodes. The term “organic” is meant to encompass both small moleculebased devices, as well as those using oligomers and polymers. Theresults show that the devices with the coating of aligned polyanilinenanofibers on the ITO surfaces have very low operation voltages comparedthe bare ITO electrodes. That is, the polyaniline lowers the operatingvoltages of the electrodes. The effects are seen for any type ofelectrodes, including both transparent and non-transparent electrodes,as well as Au, ITO and other electrodes. FIG. 17 shows operating voltagefor a ITO/PAN nanofiber/Alq3/Ca/Al electrode as a function of current.It is noted that the use of such aligned nanofibers will provide lowervoltage operation for light emitting polymer based LEDs e.g.,polyfluorene, small molecule based LEDs, e.g., Alq₃, and oligomer basedLEDs, e.g., alpha sexithiophene based LEDs.

We also demonstrate that the surface coating of aligned polyanilinenanofibers can be used as the sensors for chemicals. Sensors fabricatedby the aligned polyaniline nanofibers have very fast response when thedevices are exposed to HCl vapor or NH₄OH vapor.

Sensors based on aligned polyaniline nanofiber network can be preparedby the methods described herein. In one embodiment, a polyanilinenanofiber network was assured to be fully doped and highly conducting byexposure to 37% w/w aqueous HCl. The fiber network placed on a substratewas then exposed to vapor from a drop of 30% w/w aqueous NH₄OH placedseveral cm from the nanofiber network. The resistance of the nanofibernetwork increases over many orders of magnitude in a few seconds.Substantial increase in the nanofiber network resistance occurs in lessthan one second. In addition, field effect device based on polyanilinenanofibers in contact with source and drain electrodes have beendemonstrated, as described in the parent application.

The exemplary embodiment has been described with reference to thepreferred embodiments. Obviously, modifications and alterations willoccur to others upon reading and understanding the preceding detaileddescription. It is intended that the exemplary embodiment be construedas including all such modifications and alterations insofar as they comewithin the scope of the appended claims or the equivalents thereof.

1. A process of stretching DNA comprising the steps of: coating asurface of a substrate with aligned polyaniline or aligned substitutedpolyaniline nanofibers to form a superhydrophilic surface; introducingan aqueous DNA solution to said coated substrate; and allowing said DNAto stretch due to capillary force resulting from the superhydrophilicsurface.
 2. A process according to claim 1, further comprising the stepof depositing a layer of aligned polypyrrole on said superhydrophilicsurface.